Davood Askari

A comparative study on macrofiber composites and active fiber composites with metal-core piezoelectric actuators/sensors

Piezoelectric sensors and actuators have gained considerable interest by investigators and researchers for their use in intelligent/smart structures and electromechanical systems. Furthermore, demands from industry for sensors and actuators with higher quality and better performance for a variety of applications have lead the researchers to design piezoelectric systems with optimal configurations to enhance the performance of such actuators and sensors. An analytical micromechanics approach is presented to model the effective longitudinal mechanical properties of Metal-Core Piezoelectric Fibers (MPFs) and Macro Fiber Composites (MFCs). The model assumes general orthotropic material properties for both outside and inside materials. Next, using constitutive equations, the exact analytical solutions for the stress distributions are obtained for axially loaded active fibers. To examine the mechanical performance of the MPF and MFC, material properties and geometric dimensions are substituted into the analytical exact solutions and then effective longitudinal mechanical properties as well as the stress distributions within the domain of each constituent material are obtained and then compared. Finally, the results are presented and concluding remarks are addressed and discussed in details.

A Comparative Finite Element Analysis of Residual Stresses in Active Fiber Composites With Embedded Metal-Core Piezoelectric Fibers and Macro Fiber Composites

Residual stresses are basically developed due to intrinsic and extrinsic strains that form during the processing of composite materials. The extrinsic strains can be determined using Coefficient of Thermal Expansion (CTE), material properties, geometry of the structure, and processing conditions. Finite Element Method (FEM) as an efficient alternative technique for stress and strain analysis of the micromechanical systems and structures, has been employed to numerically investigate the residual stresses developed in Metal-Core Piezoelectric Fibers (MPF) and Active Fiber Composites (AFC) (or Macro Fiber Composites (MFC)), during the processing. Here in this work, ANSYS Finite Element Analysis (FEA) software is used to develop three different 3-dimensional models for MPF and MFC structures and then each model is solved for strain and stress results. Next, the stress and strain components of these models are studied throughout the structures to identify the magnitude and type of the stresses and strains within the constituent materials and then compared.Copyright

Analytical Modeling for Mechanical Responses of an Axially Loaded Three-Phase Active Fiber Composite

Structural integrity and stability are among the key requirements for any mechanical component used in structural applications when subjected to external or internal loads. For example, Active Fiber Composites (AFCs) or micro fiber composites (MFCs) used as sensors and actuator for vibration damping, structural control, and health monitoring in intelligent structures can be subjected to both external and internal loading. Furthermore, demands from industry for better performing sensors and actuators for the use in adaptive structures have led the researchers to investigate and design AFC/MFC systems with optimal configurations to enhance their performances. Therefore, it is essential to understand and be able to predict the mechanical behaviour of the individual AFCs/MFCs that exist in different geometrical configurations composed of various constituent materials. In this work, an analytical approach based on elasticity equations is introduced to derive exact solutions for mechanical responses (i.e., displacements, strains, and stresses) of an axially loaded threephase composite cylinder, representing an individual AFC/MFC tube. Materials of the constituents are considered to be orthotropic for no loss of generality. To validate the exact analytical solutions, finite element analysis is performed and then the results obtained from both techniques are compared where excellent agreements are achieved.Copyright

Analytical Solutions for Effective Axial Mechanical Properties of a Three-Phase Active Fiber Composite Model

Active fiber composites are among the many other components used in intelligent and smart composite structures which undergo mechanical deformation upon the application of external loads or electric fields. This work presents an analytical approach for derivations of exact solutions for the effective axial mechanical properties of active fiber composites with circular cross-sections, and while the properties of the constituent materials are considered to be generally orthotropic. First, exact analytical solutions of the effective longitudinal Young’s modulus and Poisson’s ratio are obtained for a three-phase composite cylindrical model composed of orthotropic materials. Next, Finite element analysis, as an alternative approach, is performed to numerically determine the effective axial properties of an identical three-phase composite cylinder. Finally, effective material properties obtained from analytical and finite element methods are compared to verify the derived analytical solutions. Excellent agreements are achieved between the results obtained from both techniques validating the exact analytical solutions.Copyright

Nanocomposites and Hierarchical Nanocomposites Development at Hawaii Nanotechnology Laboratory

This paper presents activities related to the development of nanocomposites and hierarchical nanocomposites; at the Hawaii Nanotechnology Laboratory of the Department of Mechanical Engineering of the University of Hawaii at Manoa. On nanocomposites, developments on toughening of polymeric materials employing nanoparticles and carbon nanotubes are reported. On hierarchical nanocomposites, first, mechanical properties improvements for continuous fiber ceramic composites using nanoparticles are discussed. Second, a multifunctional micro-brush using carbon nanotubes is discussed. Third, the structure of a micro-foam using carbon nanotubes is explained. Finally, the multifunctional properties improvement of a novel three-dimensional hierarchical nanocomposite employing carbon nanotubes is discussed. In closing, the effect of chirality of single-walled nanotubes on their thermomechanical properties evaluated analytically using asymptotic homogenization method and numerically employing finite element method will be explained, and analytical closed form solutions for matrix filled nanotube nanocomposites, also verified numerically, assuming generally cylindrical orthotropic properties will be reported.© 2006 ASME

Mechanical Performance of Matrix Filled Single-Walled Carbon Nanotube Reinforced Nanocomposites

It is frequently reported that carbon nanotubes can efficiently be used to reinforce composite materials and considerably improve their structural mechanical properties. Therefore, it is essential to investigate the effective properties of such nanocomposites. In this work, an analytical approach is employed to derive the analytical exact solutions for the effective Young’s modulus and major Poisson’s ratio of a three-phase composite cylinder model representing a matrix filled single-walled carbon nanotube (SWCNT) embedded in another host material. In this study, all three constituents are considered generally cylindrical orthotropic. For validation, results from finite element analysis of an identical 3-D model are compared to those obtained analytically. It is shown that both techniques are in excellent agreement and therefore analytical exact solutions for the prediction of effective axial Young’s modulus and major Poisson’s ratio of the filled SWCNT embedded in another host material and all having orthotropic properties are verified.Copyright

Analytical Solutions for Effective Axial Mechanical Properties of Active Fiber Composites

This work presents analytical solutions for the effective axial mechanical properties of the active fiber composites with different geometries, e.g., circular and rectangular cross-sections, and while the properties of the constituent materials are considered to be generally orthotropic. Analytical exact solutions of the effective longitudinal Youngs moduli and major Poissons ratios are obtained for two different geometries of a 2-phase composite model composed of orthotropic materials. Finite element analyses (FEAs) are also performed to verify the obtained analytical exact solutions. Excellent agreements are achieved between the results obtained from both techniques. Finally, the effective mechanical properties calculated from analytical solutions for the modeled geometrical configurations are compared.Copyright

Chirality Effects on Axial Thermomechanical Properties of Carbon Nanotubes

The nearly one dimensional carbon nanotubes with their novel physical and mechanical properties have received ever increasing attention in recent years for the use in a wide range of applications in which semiconductor nano-structures, nano-devices/sensors, and nano-electro-mechanical systems are to be integrated. However, carbon nanotubes exist in various chirality configurations each of which may perform differently when they are subjected to external mechanical and thermal loads, temperatures changes, and magnetic fields. Therefore, a detailed and fundamental investigation of the effects of chirality angles on thermomechanical performance of carbon nanotubes is needed to explain the behavior of such structures. Here in this work, finite element method (FEM) is employed to numerically investigate the responses of carbon nanotubes to external mechanical loads and temperatures changes. Single-walled carbon nanotubes (SWCNTs) with different chirality configurations, i.e., zigzag, armchair, and chiral are modeled and their effective thermomechanical properties are investigated. Finally, results are discussed and compared with the existing results from literature.Copyright

Mechanical Properties Predictions and Responses of Defected Carbon Nanotubes Subjected to Axial Loading

The increasing demand for fabrication of smaller structural and electronic devices with higher performance such as NEMS/MEMS has created great interest and motivation for extensive research and investigations in nanotechnology and its applications. Unique mechanical, thermal, and electrical properties of the one dimensional carbon nanotubes (CNTs) structures project CNTs as an excellent candidate for the future NEMS/MEMS devices. However, carbon nanotubes do not always exist in their perfect hexagonal lattice structures. Defects may appear during the purification stages or chemical treatments as it might even be desirable for functionalization of carbon nanotubes. On the other hand, defects can greatly influence the mechanical performance of carbon nanotubes in structural applications where they are subjected to external mechanical loads. Therefore, a detailed investigation of the effects of defects on mechanical performance of carbon nanotubes is needed to explain the behavior of such structures. Here in this work, finite element method (FEM) is employed to numerically investigate the responses of defected carbon nanotubes to external loads. Single-walled carbon nanotubes (SWCNTs) with different structural configurations, i.e., zigzag, armchair, and chiral, with different types of vacancy defects are modeled and their effective mechanical properties are investigated. Finally, results are discussed and compared with those obtained for SWCNTs without defects.© 2006 ASME

Mechanical Performance Analysis of Axially Loaded Active Fiber Composites

Active fiber composites (AFCs) or macro fiber composites (MFCs) are often subjected to external loads and mechanical deformations. Furthermore, demands from industry for sensors and actuators with higher quality and better performance for specific applications have lead the researchers to design piezoelectric systems with optimal configurations to enhance the performance of such actuators and sensors. Therefore, it is important to investigate the mechanical performance of the individual AFCs/MFCs that exist in different geometrical configurations with alternative constituent materials. Here in this work, analytical exact solutions for effective mechanical properties of AFCs/MFCs are derived and then used to obtain the analytical exact solutions for displacements, strains and stresses induced in axially loaded AFCs/MFCs, which is the most common loading condition, i.e., tension/compression, that exist for such structures. In our study, constituent materials are considered orthotropic and two different geometries, i.e., AFCs/MFCs with circular and rectangular cross-sections, are investigated. For the given material properties, the displacement, strain, and stress results, corresponding to an axially applied external load are obtained in the domain of each constituent for both AFC/MFC geometrical configurations. To verify the analytical exact solution, 3-dimensional finite element analysis is performed and then the results obtained from both techniques are compared where excellent agreement was achieved.Copyright